1. Field of the Invention
The present invention relates to an optical head apparatus for storing information in a storage medium such as an optical medium or a magneto-optical medium like an optical disk or an optical card and reading out or erasing the stored information from the optical memory. Also, the present invention relates to an optical information reproducing method and an optical information apparatus for optically reading information from the storage medium with the optical head apparatus.
2. Description of the Related Art
An optical memory technique has been put to practical use to manufacture an optical disk in which a pit pattern indicating information is formed. The optical disk is utilized as a high density and large capacity of storage medium. For example, the optical disk is utilized for a digital audio disk, a video disk, a document file disk, and a data file disk. To store information in the optical disk and to read the information from the the optical disk, a light beam radiated from a light source is minutely narrowed in diameter by an optical system, and the light beam minutely narrowed is radiated to the optical disk through the optical system. Therefore, the light beam is required to be reliably controlled by the optical system with high accuracy.
A major part of the optical system is occupied by an optical head apparatus, and a fundamental function of the optical head apparatus is classified into converging performance for minutely narrowing a light beam to form a small diffraction-limited spot of the light beam radiated on an optical disk, focus control in a focus servo system, tracking control in a tracking serve system, and detection of pit signals obtained by radiating the light beam on the pit pattern of the optical disk. The fundamental function of the optical head apparatus is determined by the combination of optical sub-systems and a photoelectric transfer detecting process according to a purpose and a use.
Specifically, an optical pick up head apparatus in which a holographic optical element (or hologram) is utilized to minimize and thin the optical pick up apparatus has been recently proposed.
Previously Proposed Art
FIG. 1 is a constitutional view of a conventional optical head apparatus proposed in Japanese Patent Application No. 46630 of 1991.
As shown in FIG. 1, a conventional optical head apparatus 11 is provided with a light beam source 12 such as a semiconductor laser, a reflection type of blazed hologram 13 for diffracting and reflecting a light beam radiated from the light beam source 12, an information medium 14 such as an optical disk for storing information indicated by patterned pits, an objective lens 15 for converging the light beam diffracted and reflected at the information medium 14 to read the information, and a photo detector 16 for detecting the intensity of the light beam reflected on the information medium 14.
In the above configuration, a light beam 17 (or a laser beam) radiated from the light beam source 12 is radiated to the hologram 13, and zero-order diffraction light 18 is mainly reflected to an outgoing optical path. Thereafter, the zero-order diffraction light 18 passes through the object lens 15 and is converged at the information medium 14. In the information medium 14, the information stored is read by the zero-order diffraction light 18. Thereafter, the zero-order diffraction light 18 is fed back to an incoming optical path which is the same as the outgoing optical path of the light 18. Therefore, the zero-order diffraction light 18 is radiated to the hologram 13 after passing through the objective lens 15. Thereafter, first-order diffraction light 19 is mainly diffracted and reflected in the hologram 13, and is radiated to the photo detector 16.
In the photo detector 16, the intensity distribution of the first-order diffraction light 19 is detected. Therefore, a servo signal for adjusting the position of the objective lens 15 is obtained. Also, the intensity of the first-order diffraction light 19 is detected in the photo detector 16. Because the information medium 14 is rotated with high speed, the patterned pits radiated by the light 17 are changed so that the intensity of the first-order diffraction light 19 detected is changed. Therefore, an information signal indicating the information stored in the information medium 14 is obtained by detecting the change in intensity of the first-order diffraction light 19.
In the above operation, unnecessary diffraction light such as first-order diffraction light and minus first-order diffraction light necessarily occurs in the hologram 13 when the light beam 17 is radiated to the hologram 13 in the outgoing optical path. The unnecessary diffraction light of the outgoing optical path also reads the information stored in the information medium 14, and the unnecessary light is radiated to the photo detector 16. To prevent the occurrence of the unnecessary light, the hologram 13 is manufactured to form a blazed hologram pattern on the surface thereof, so that the intensity of the unnecessary light radiated to the photo detector 16 is decreased.
Next, a method for manufacturing the blazed hologram 13, in which a blazed hologram pattern is formed, is described with reference to FIGS. 2A to 2F.
As shown in FIG. 2A, a hologram substrate 21 is coated with a resist 22, and the resist 22 is covered with a first patterned photomask 28. Thereafter, the resist 22 is exposed to ultraviolet radiation to transfer a first pattern to the resist 22. After the photomask 23 is taken off, the resist 22 exposed is developed as shown in FIG. 2B. After development, the hologram substrate 21 exposed is etched with an etchant at a depth H1 in a first etching process, and the resist 22 is stripped as shown in FIG. 2C.
Thereafter, the hologram substrate 21 etched is again coated with a resist 24, and the resist 24 is covered with a second patterned photomask 25 as shown in FIG. 2D. Thereafter, the resist 24 is exposed to ultraviolet radiation to transfer a second pattern to the resist 24. After the photomask 25 is taken off, the resist 24 exposed is developed as shown in FIG. 2E. After development, the hologram substrate 21 exposed is again etched with an etchant at a depth H2 in a second etching process, and the resist 24 is stripped as shown in FIG. 2F.
Accordingly, as shown in FIG. 3, the brazed hologram 13 of which the surface is formed in echelon shape can be manufactured by etching the hologram substrate 21 twice, so that degree of freedom for controlling the diffraction efficiency in the brazed hologram 13 can be enhanced. In detail, a first phase-modulation degree .phi.1 of the light beam 17 determined by the depth H1 and a second phase-modulation degree .phi.2 of the light beam 17 determined by the depth H2 are controlled independently, so that the diffraction efficiency for the unnecessary diffraction light can be lowered. In contrast, an intensity ratio of the first-order diffraction light 19 to the light beam 17 can be enhanced.
In addition, because the brazed hologram 13 functions as a mirror for bending the outgoing and incoming optical paths, a thin type of optical head apparatus can be manufactured with a small number of parts.
Next, adverse influence of the unnecessary diffraction light occurring in the outgoing optical path on the information signal obtained in the photo detector 16 is described.
For example, minus first-order diffraction light occurring in the hologram 13 in the outgoing optical path is reflected by the information medium 14, and the minus first-order diffraction light reflected is radiated to the hologram 13. In the hologram 13, zero-order diffraction light of the incoming optical path occurs. Thereafter, the zero-order diffraction light is radiated to the photo detector 16. As shown in FIG. 4, in general, N-order (N=-1, 0, 1, 2, - - - ) diffraction lights 41 of the outgoing optical path occurring in the hologram 13 are reflected by the information medium 14, and the N-order diffraction lights 41 reflected are radiated to the hologram 13. Thereafter, (N+1)-order diffraction lights 42 of the incoming optical path occur, and the (N+1)-order diffraction lights 42 are radiated to the photo detector 16.
In this case, first-order diffraction light (N=0) radiated to the photo detector 16 is the largest in intensity among the (N+1)-order diffraction lights. Therefore, first-order diffraction light L1 of the incoming optical path occurring in the hologram 13 is utilized as necessary light in the conventional optical head apparatus 11. The second largest light in intensity is zero-order diffraction light L2 (N=-1) of the incoming optical path occurring by diffracting minus first-order diffraction light of the outgoing optical path in the hologram 13 and second-order diffraction light L3 (N=1) of the incoming optical path occurring by diffracting zero-order diffraction light of the outgoing optical path in the hologram 13.
Therefore, in cases where the conventional optical head apparatus 11 is designed so as to sufficiently decrease the ratio in intensity of both the zero-order diffraction light L2 and the second-order diffraction light L3 to the first-order diffraction light L1, the adverse influence of the unnecessary light including the diffraction lights L2, L3 on the information signal can be minimized. That is, noise included in the information signal can be reduced.
In addition to the intensity reduction of the lights L2, L3, because aberration in the N-order diffraction lights becomes larger as the number N is higher, converging spots of the N-order diffraction lights on the information medium 14 become larger in size as the number N is higher. Therefore, the intensity of higher order diffraction light such as third-order diffraction light radiated to the information medium 14 rotated does not vary so much, so that the information stored in the information medium 14 is not read by the higher order diffraction light. As a result, even though the higher order diffraction light is radiated to the photo detector 16, any noise is not generated in the information signal so much.
Accordingly, it is enough to reduce the intensities of the diffraction lights L2, L3 to minimize the noise included in the information signal.
The relationship between the second phase-modulation degree .phi.2 of the light beam 17 determined by the depth H2 and a ratio E.sub.2 in intensity of the unnecessary light L2, L3 to the first-order diffraction light L1 is graphically shown in FIG. 5.
As shown in FIG. 5, in cases where the second phase-modulation degree .phi.2 of the light beam 17 ranges from 0.2.pi. to 0.45.pi., the ratio E.sub.2 is less than 0.3.
Problems to be Solved by the Invention
However, the ratio E.sub.2 is over 0.2 so that a small quantity of zero-order diffraction light L2 and a small quantity of second-order diffraction light L3 are inevitably radiated to the photo detector 16. Therefore, not only a required piece of information stored in the information medium 14 is mainly transferred to the photo detector 16 as the information signal by the first-order diffraction light L1, but also an unnecessary piece of information is secondarily transferred to the photo detector 16 as the noise in small quantity by the zero-order diffraction light L2 and the second-order diffraction light L3.
Accordingly, even though the blazed hologram 13 is manufactured to reduce the intensities of the unnecessary lights L2, L3 as shown in FIGS. 2, 3, the noise are inevitably generated in the photo detector 16.